The present invention relates to generally an igntion timing control device for an engine and more particularly an ignition timing control device of the type which automatically controls the ignition timing such that the emission of pollutants such as nitrogen oxides (NOx) and unburned hydrocarbon (HC) is decreased only when the temperature of the engine is within a predetermined temperature range.
It has been well known that the retardation of ignition timing can effectively reduce the emission of pollutants such as nitrogen oxides and hydrocarbons. In order to retard the ignition timing, there has been proposed a method for opening a negative pressure line between a vacuum advance mechanism of a distributor and a carburetor to the surrounding atmosphere. However this method has defects in that the automotive engine output drops when the engine temperature is low and the engine becomes overheated when the load is increased. It is well known that the emission of nitrogen oxides is relatively low when the engine temperature and hence the engine output are low. It is therefore preferable to activate a vacuum advance mechanism until the engine temperature rises to such a level that sufficient output may be obtained and to deactivate the vacuum advance mechanism when the engine temperature rises to such a point that a large quantity of nitrogen oxides is produced and discharged. Furthermore it is clear that the overheating of an engine due to the deactivation of the vacuum advance mechanism may be prevented when the vacuum advance mechanism is activated again.